With EK3_RNG_USE_HGT, Copter uses a range finder that reads on the ground
for height while taking off, so the baro offset filter runs while the
motors spool up and learns the prop-wash baro error as offset. ALT_HOLD's
takeoff ends as the vehicle leaves the ground, the source goes back to baro
there, and the offset carries the error into the flight: in SITL with
SIM_BARO_GEFF_M 3 the EKF height read 2.5 m high from then until landing.
While takeoff or touchdown is expected, do not update the offset, and do
not reset the height to the baro on switching to it. Either alone is not
enough: with only the offset held, the reset drops the height by the whole
error at liftoff (3.0 m); with only the reset skipped, the offset still
carries it (2.5 m). With both, -0.17 m worst in the takeoff and 0.03 m mean
above 5 m.
Baro drift is not learned while the flags are set, which on Copter
includes the whole time it sits armed on the ground. Learning only the
upward part of the error was tried: in SITL it did no better, and baro
noise ratchets it upwards (0.07 m after a 10 s armed wait with
SIM_BARO_RND 0.2). Fixed wing sets the flags for a launch rather than for
rotor wash, so it is left as it was.
Andy Piper
bb615f0f93